Science 7–10 · Year 7
Weight and mass: hanging known masses from a newton meter
Physical sciences — Forces (NSW Stage 4 focus area)
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The idea
Weight is the gravitational force on a mass, so the newton-meter reading rises in direct proportion to the mass hung from it and the gradient is the gravitational field strength.
What you need
- 0 to 10 N spring newton meter, 1
- slotted mass set 100 g to 500 g with hanger (50 g hanger plus 50 g slots), 1
- retort stand, boss and clamp, 1
- electronic balance reading to 1 g, 1 per class
How to do it
- Check the newton meter reads zero with nothing on the hook; adjust the zero screw if it does not.
- Weigh the hanger and each slotted mass on the balance and record the mass in grams, then convert to kilograms.
- Hang the 100 g hanger-and-slot combination on the newton meter and read the force in newtons to the nearest 0.1 N. Record it.
- Add slots to reach 200 g, 300 g, 400 g and 500 g, reading the force each time. Repeat the whole set twice more and average the three readings for each mass.
- Plot force (N) on the vertical axis against mass (kg) on the horizontal axis and draw the line of best fit through the origin.
- Calculate the gradient in newtons per kilogram and compare it with 9.8 N/kg.
What you should see
The points lie on a straight line through the origin. With g = 9.8 N/kg the 0.500 kg load reads 4.90 N and the 0.100 kg load reads 0.98 N, so the gradient should be close to 9.8 N/kg. A reading uncertainty of 0.1 N on the 0.500 kg load is 0.2 N/kg in the gradient (0.1 N / 0.500 kg), so a gradient within that of 9.8 N/kg agrees with the accepted value. The learner knows it worked when doubling the mass doubles the reading within 0.2 N.
What changes
- What you change
- mass hung on the hook (kg)
- What you measure
- newton-meter reading (N)
- What you keep the same
- the same newton meter
- meter held vertically and read at eye level
- load hanging still before reading
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Weight and mass are the same quantity.
- A heavier object has weight because of its size rather than because gravity pulls on its mass.
- The newton meter measures mass in kilograms.
Safety card
Hazards
- masses falling on feet
- over-stretching the spring
Controls
- work over the bench, not the floor
- never exceed the meter's marked range
Note
No hazardous chemicals or naked flames are used. Complete the school's risk assessment for the activity before the lesson; the NSW Department of Education Science safety and compliance page points to CSIS 1.7 (Risk assessment – a pre-requisite for risk control) for how to carry it out.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC4-FOR-01SC4-WS-01SC4-WS-05
- Australian Curriculum v9AC9S7U04AC9S7I03AC9S7I04
Sources
The pages the author read to write this activity.